Organic molecule-based optical technology components such as electro-optic modulators promise greatly increased rates of information transmission by enhancing network speed, capacity, and bandwidth for data networking and telecommunications. There is a vast need for increased data handling density in photonic devices, and future high-speed fiber-optic networks will be required to carry orders of magnitude more data than possible with conventional electronic systems and to easily handle phone calls, e-mail, webpages, video, and high-definition television (HDTV) signals. Therefore, the development of new electro-optic/second-order nonlinear optical (NLO) materials with excellent optical, thermal, and chemical properties is a topic of great current scientific interest. Of the synthetic approaches investigated, Langmuir-Blodgett (LB) film transfer, polymer poling, and self-assembly (SA) have been used to obtain soft thin films with a variety of electro-optic response properties.
Intrinsically acentric SA organic materials can exhibit far higher electro-optic coefficients and lower dielectric constants than established inorganic materials (e.g., lithium niobate and potassium dihydrogen phosphate), and do not require electric field poling. (For reviews see the Chem. Rev. special issue on Optical Nonlinearities in Chemistry, ed: D. M. Burland, 1994, 94, 1-278.) Chemisorptive siloxane SA was originally developed by Sagiv (R. Moaz, J. Sagiv, Langmuir 1987, 3, 1034-1044) and is known to yield robust, densely packed organic films on hydroxylated surfaces. Self-assembled mono- and multilayered structures are accessible with relatively simple hydro- or fluorocarbon chains (A. Ullman, An Introduction to Ultrathin Organic Films: from Langmuir-Blodgett to Self-Assembly, Academic Press, Inc. San Diego, 1991), whereas fabrication of complex superlattices is relatively rare. For instance, Katz, et al. reported the formation of acentric multilayers by alternately depositing layers of phosphonate-substituted azo chromophores and zirconyl salts. (H. E. Katz, W. L., Wilson, G. Scheller, J. Am. Chem. Soc., 1994, 116, 6636-6640.)
Previous studies showed that robust, acentric mono- and multilayered structures composed of stilbazolium and related chromophores (See, FIGS. 1A-C) and exhibiting very large NLO response properties (χ(2)=150-200 pm/V) can be obtained by a three-step procedure, the second of which involves a cumbersome spin-coating of chromophore solutions, followed by vacuum oven treatment. The synthetic tools available for the formation of surface-bound functional organic multilayered structures are rather limited in comparison to the tremendous variety of reactions known in solution phase organic chemistry.
Organic monolayers containing nonpolar end-groups are chemically inert towards binding of chloro- or alkoxy silanes from the solution phase. However, regeneration of a new reactive hydroxylated or carboxylated surface is an essential requirement for the iterative growth of siloxane-based multilayers. In order to form highly ordered multilayered structures, a constant and/or a large density of reactive sites must be present at the surface of each added layer. To this end, it was recently demonstrated that 9-fluorenylmethoxycarbonyl (Fmoc) and di-p-methoxytrityl (DMT) groups can be used for the reversible protection of amine and hydroxyl-terminated alkanethiol monolayers on gold substrates. (Frutos, J. M. Brockman, R. M. Corn, Langmuir 2000, in press.) Hydroboration or oxidation of terminal double bonds (L. Netzer, J. Sagiv, J. Am. Chem. Soc. 1983, 105, 674-676; R. Moaz, S. Matlis, E. DiMasi, B. M. Ocko, J. Sagiv, Nature 1996, 384, 150), hydrolysis of phosphonate esters (G. A. Neff, C. J. Page, E. Meintjes, T. Tsuda, W. C. Pilgrim, N. Roberts, W. W. Warren, Jr., Langmuir 1996, 12, 238-242), reduction of methyl esters (S. R. Wasserman, Y. T. Tao, G. M. Whitesides, Langmuir 1989, 5, 1074-1087; M. Pomerantz, A. Segmuller, L. Netzer, L. Sagiv, J. Thin Solid Films 1985, 132, 153-162), and photolysis of organic thin films (R. J. Collins, I. T. Bae, D. A. Scherson, C. N. Sukenik, Langmuir 1996, 12, 5509-5111) have also been used to create hydroxyl-terminated surfaces.